This invention relates generally to a method of driving an electro-optical device, and more particularly to a method of driving a display unit or a transmitting light control unit or the like using a liquid crystal panel which includes a hysteresis-type super-twisted nematic mode of operation.
A wide variety of electro-optical devices are presently available. One type of electro-optical device includes a liquid crystal panel of relatively simple construction which is commonly used as a display unit, a photoshutter or the like due to its relatively small size, light weight and minimal electric power consumption requirements.
Liquid crystal electro-optical devices having large display capacities and areas are far simpler to manufacture when using simple rather than an active matrixes. Nevertheless, conventional liquid crystal electro-optical devices which have large display capacities using simple-matrices are difficult to manufacture due to their optical response characteristics. More particularly, the display quality and/or response velocity deteriorates due to a lack of desirable electro-optical characteristics within the liquid crystal. Such deterioration has been observed in an electro-optical device using a twisted nematic liquid crystal having a 1/200 duty as well as a super twisted nematic liquid crystal (i.e. a STN/SBE mode) where high performance is attained by providing large twist angles and having a 1/400 or greater duty.
In an attempt to overcome the aforementioned deterioration in display quality and/or in response velocity, a method in an article by Philips Research Laboratories of Eindhoven, The Netherlands appearing in the J. Appl. Phys. 59(9), May 1, 1986 at pages 3087-3090, published by the American Institute of Physics, proposes application of a bistable voltage for storing data within a liquid crystal whose orientation is controlled. This method takes advantage of the fact that the applied voltage and transmissivity or reflectivity are combined to cause a hysteresis effect in a liquid crystal whose angle of twist exceeds 90.degree. depending on the material of the liquid crystal. These hysteresis effects are shown in FIGS. 2(a) and 2(b). The information selectively written during the writing process is held in an associated pixel by providing a holding voltage to a corresponding scan line of the matrix. The value of the holding voltage is set to fall within the hysteresis loop of the voltage-transmissivity or voltage-reflectivity curves as shown in FIGS. 2(a) and 2(b). The display mode associated with the voltage-transmissivity or voltage-reflectivity curve is hereinafter referred to as a hysteresis-type super-twisted nematic mode (HTN mode).
The writing and erasing processes of the Philips system occur over approximately 0.2 milliseconds and 50 milliseconds, respectively, presenting an extremely asymmetric relationship therebetween. Consequently, the driving method of Philips can not individually and selectively erase the data stored in each row of pixels and then rewrite new data into each row of pixels. Rather, the contents of the display is renewed by selectively writing data into each row of pixels after collectively erasing the data from the entire screen of pixels.
Electro-optical devices using the HTN mode must satisfy various requirements to produce the desired hysteresis characteristics. Hysteresis characteristics vary depending on such factors as twist angle, cell gap, pre-tilt angle within the liquid crystal panel of the electro-optical device, spontaneous pitch, elastic constants, dielectric constants of the liquid crystal composition and interaction between the orientation film (i.e. interface regulating force) and liquid crystal molecules. Typically, the pre-tilt angle, twist angle, interface regulating force, and elastic constant ratios of K33/K11 and K33/K22 are relatively large. The ratio of dielectric anisotropies represented by .DELTA..epsilon./.epsilon..perp. and deviation of .DELTA.P=Pc/Ps-1 are generally relatively small. Pitch Pc is determined by orientation processing of a liquid crystal cell. Pitch Ps represents the spontaneous pitch of the liquid crystal composition.
As shown in FIG. 3, the driving method used in an electro-optical device employing a conventional HTN driving mode is based on maintaining a scan line corresponding to a pixel whose contents is to be erased at substantially zero volts during an erasing period t.sub.e. Erasing period t.sub.e immediately follows a holding time t.sub.h1 during which time data from a current frame of the display is held in the pixels by providing a holding voltage .+-.V.sub.h. The value of holding voltage .+-.V.sub.h is set based on the voltages within the hysteresis loop of the voltage-transmissivity or voltagereflectivity curve. Data provided on a data line is written into the pixel during a writing time t.sub.s by providing a writing pulse having a width P.sub.w on a corresponding scan line. Erasing period t.sub.e of the current frame immediately precedes writing time t.sub.s of the next frame of the display.
Multiplex driving in which a conventional HTN driving method is employed updates the display contents of a matrix of pixels forming a complete liquid crystal panel through batch erasing followed by a writing process using sequential scanning. More particularly, a conventional HTN driving method collectively erases the entire display at one time and then rewrites the display through sequential scanning. The time during which data is displayed by pixels associated with the initial scan line selected as opposed to pixels associated with the last scan line selected is significantly different. Therefore, erasing of that portion of the display associated with the last stage of the scanning period is conspicuous.
For example, in a conventional HTN driving method in which writing time t.sub.s is 0.2 milliseconds per line and erasing period t.sub.e is 50 msecs, a display having 1000 lines (i.e. rows of pixels) will result in the first row of pixels having data written therein after 50 msecs from the initiation of erasing period t.sub.e. The last row of pixels will have data written therein after 250 msecs from the initiation of erasing period t.sub.e. Similarly, for a screen having 500 lines, the 500th line will have data written therein after 150 msecs from the initiation of erasing period t.sub.e. The delay between writing of data between the first and last line of the screen coupled with erasing of the entire screen at one time (i.e. batch erasing) creates a flashing (i.e. flickering) effect on the screen especially prominent around those pixels associated with the last scan lines to be selected. The more lines on the display, the more prominent will be the flashing. In other words, the conventional HTN driving method requires lines scanned during the latter part of the scanning period to appear without data (i.e. blank) for longer periods of time than lines scanned during the initial stages of the scanning period.
Where rewriting of each line of the display is based on providing a minimum erasing time and a reasonable interval between writing times on successive lines, the lines associated with the end of the scanning period will appear to display data for a relatively short period of time compared to lines associated with the beginning of the scanning period. A decline in the display quality (i.e. non-uniform display density) results.
Accordingly, it is desirable to provide a method for driving a liquid crystal type electro-optical device employing a HTN mode in which data is stored on all lines of the display for substantially the same period of time resulting in a display which avoids a flashing effect on the screen.